1. Field of the Art
In the search for new oil and natural gas pools, it is necessary to sink increasingly deeper bores and to drill through increasingly more difficult formations. Accordingly, currently used drilling fluids must satisfy more and more stringent requirements, particularly with respect to temperature and electrolyte stability. In addition, increasing attention is being paid to ecological considerations in the use of such fluids and in their subsequent disposal.
2. Discussion of Related Art
Water-based drilling fluids based on bentonite or bentonite in combination with polysaccharides as thickening auxiliaries are often used today for comparatively shallow wells. Typical polysaccharides used with bentonite include starch, carboxymethyl cellulose (CMC), carboxymethyl hydroxyethyl cellulose (CMHEC), and hydroxyethyl cellulose (HEC); these polysaccharides, however, are only stable to about 120.degree. to 150.degree. C. Accordingly, for working at the relatively high well temperatures associated with deep bores, it is customary today either to use drilling fluids based on mineral oils or to use water-based systems containing special modifying synthetic organic polymers which are more temperature-stable than the prior art polysaccharides. The development of high-stress water-based drilling fluids has resulted in a number of proposed polymers as thermally stable thickeners for these fluids, especially for use with swellable clays, and, more especially, for use with a variety of modified bentonites. Typical polymers for this use are described in the following publications: German patent applications 31 44 770, and 34 04 491; and European patent application Nos. EP 0 122 073 and EP 0 131 124. All these water-based drilling fluid systems have, however, distinct weaknesses at temperatures above 200.degree. C., particularly in the presence of high concentrations of alkaline earth metal ions.